Binary sensors are generally understood to be those sensing devices whose output exists in only one of two states, on or off (High or Low). The simplest example is a mechanical switch; other examples include but are not limited to photoelectric sensors, inductive proximity sensors, capacitive sensors, and ultrasonic sensors. Binary sensors are used in high quantities in factory automation, in most cases as input devices to Programmable Logic Controllers ("PLCs"). Typically, a binary sensor utilizes three wires to be connected to the PLC: positive supply voltage (+V), negative supply voltage (-V), and the sensor output signal (OUTPUT). In installations with large numbers of binary sensors, wiring costs and the PLC input port costs can be greater than the cost of the sensors themselves.
As a consequence, several data bus systems have been developed that permit binary sensors (and binary load such as solenoids) to operate over a single pair of datacom wires plus DC power, and to connect directly to the backplane of many PLC types. Examples of such data bus systems include Seriplex.TM. by Square D Company, ASI.TM. by Siemens, SDS.TM. by Microswitch, LON by Echelon and DeviceNet.TM. by Allen Bradley. Each of these systems has certain advantages over the others, and to date no single bus protocol for binary sensors has emerged. Thus customers and suppliers must find ways to operate with the above-mentioned buses each having its own distinctive protocol.
In order for a binary sensor to operate over a single pair of datacom wires, additional bus circuitry must be used in conjunction with the binary sensor. The additional circuitry provides a means for the binary sensor to be modified so that its communications protocol is compatible with that of the bus network system for communication. There are two philosophies regarding where to put the required bus network circuitry itself; one is that it should reside in the binary sensor, and the other is that it should reside in a separate junction box. The argument for placement of the bus circuitry inside the binary sensor is that there are already electronic circuits inside the sensor, and it does not add much to the overall cost of the sensor to add such extra circuitry for the bus interface. Accordingly, a binary sensor including the bus network circuitry would utilize a separate junction box that does not include or need the bus network interface circuitry. Such a junction box would merely comprise general interconnection circuitry.
On the other hand, each binary sensor manufacturer has thousands of different sensor models, and to include the required bus network interface circuitry in each binary sensor for each bus type would more than quadruple the size of the product line offering. Thus, a binary sensor that does not include the bus network interface circuitry would be used in conjunction with a junction box that includes such bus network interface circuitry. Inclusion of the bus network interface circuitry in the junction box would allow all permutations of the sensor types and bus types, and thereby cut down on the size of the product line offering. Because the cost of a junction box including bus network interface circuitry can be amortized over all of the inputs in a multiple input junction box, it is the least expensive approach when all junction box inputs are utilized. On the other hand, if the binary sensors are long distances from one another, as on a conveyor system, the multiple inputs negates the advantages of a bus system if wires from the sensors must be run long distances to the nearest junction box. In that situation, it is desireable to have the bus circuitry built into the sensor and to use junction boxes that do not include bus network interface circuitry.
The present invention overcomes the above difficulties by providing a method of easily accommodating both of the above described scenarios. It is a sensor that can be modified by a user to include bus network interface circuitry or adapted for use with a junction box including the bus network interface circuitry. The sensor can be further modified for communication with a number of different bus network systems, whichever network system a customer might select.